The present invention relates to a composite video signal processing circuit for the NTSC color television system, more particularly, to a luminance/chromaticity signal separation circuit capable of exactly separating a luminance signal and a chromaticity signal by selectively using output signals of three line comb filters which are adapted to vertical luminance variations. FIG. 1 shows a conventional luminance/chromaticity signal separation circuit for a composite video signal comprising
one-line delayers 1a-1d for sequentially delaying a composite video signal CV by one horizontal line,
multipliers 4a-4c for multiplying output signals of the one-line delayers 1a-1c,
multipliers 4d-4h for multiplying said composite video signal CV and the output signals of the one-line delayers 1a-1d respectively,
adders 2a, 2b and 2c for adding output signals of said multipliers (4a-4c), (4f-4h) and (4d-4f), respectively,
inverters 3a and 3b for inverting output signals of the adders 2b and 2c,
subtractors 5a-5c for subtracting the output signals of said one-line delayers 1b-1d from the output signals of said one-line delayers 1a and 1b and said composite video signal CV,
a minimum value detector 6 for detecting and generating as a select control signal the smallest one of the output signals of the subtractors 5a-5c,
a selector 7 for selecting and outputting the output signal of said adder 2a and the output signals of said inverters 3a and 3b in accordance with the selection control signal of the minimum value detector 6,
a band-pass filter for generating a chromaticity signal C with removing direct current components from the output signal of the selector 7, and
a subtractor 9 for generating a luminance signal Y by subtracting said chromaticity signal C from the output signal of said one-line delayer 1b.
The operations of the conventional circuits will be described in detail as follows.
The composite video signal CV is sequentially generated with delaying by one line by one-line delayers 1a-1d, the output signals of the one-line delayers 1a-1c. Those out-puts are multiplied by -1/4, 1/2, -1/4 by the respective multipliers 4a-4c and are added in the adder 2a. Then the signal outputted from adder 2a is applied to the terminal a of the selector 7. The output signals of the one-line delayers 1b-1d multiplied by -1/4, 1/2, -1/4 by the respective multipliers 4f-4h are added in the adder 2b, and the output signal of the adder 2b is inverted by the inverter 3a and then applied to the terminal b of the selector 7. And also, the composite video signal CV and the output signals of the one-line delayers 1a and 1b are multiplied by -1/4, 1/2, -1/4 by the respective multipliers 4d-4f and then added by the adder 2c. The output signal of the adder 2c is inverted by the inverter 3b and then applied to the terminal c of the selector 7.
In the meantime, the composite video signal CV from which the output signal of the one-line delayer 1b has been subtracted in the subtractor 5a is applied to the minimum value detector 6. The output signals of the one-line delayers 1a and 1b, from which the output signals of the one-line delayers 1c and 1d have been subtracted by subtractors 5a and 5c, respectively are applied to the minimum value detector 6.
As a result, the minimum value detector 6 detects and generates as a selection control signal the minimum one of the output signals of the subtractors 5a-5c. The selection control signal selects one of the output signals of the adder 2a and the inverters 3a and 3b from the selector 7.
Such a signal selected from the selector 7 is generated as a chromaticity signal c after removal of direct current components through the band-pass filter 8. The output signal of the one-line delayer 1b from which said chromaticity signal has been subtracted in the subtractor 9 appears as a luminance signal Y.
The composite video signal for the NTSC color television system in the form of a suppressed carrier modulation of a chrominance signal to a chrominance subcarrier is added to a luminance signal. The chrominance subcarrier has a phase difference of 180.degree. between adjacent lines.
Even though the above-mentioned conventional circuit separates the luminance signal and the chromaticity signal by using characteristics of the composite video signal and the correlation between the adjacent lines, it has problems that, in case of vertical luminance variations as well as in case of a small variation of the luminance signal, an exact separation of the luminance signal and the chromaticity signal is not made because the correlation between adjacent lines is not obtained. Also, its configuration becomes complicated with use of a plurality of one-line delayers.